Abstract:

The present invention provides a technique of suppressing the generation
of radio noise in a spark plug, wherein a high dielectric constant
fixation-assisting member, formed of a high dielectric constant material
which is higher in dielectric constant than alumina, is provided between
a metallic shell and a second conductive portion CP2 which includes a
metallic terminal of the spark plug 100.

Claims:

1. A spark plug comprising:a center electrode,a terminal portion
electrically connected to the center electrode so as to apply a voltage
from an external power source to the center electrode;a ground electrode
disposed on a front end side such that a gap for spark discharge is
formed between the ground electrode and the center electrode;a metallic
shell which holds the ground electrode, which is electrically connected
to the ground electrode, and in which the center electrode is disposed;
andan insulator which is held within the metallic shell and in which the
center electrode and the terminal portion are held, whereinthe insulator
includes a collar portion which is located at an approximate center with
respect to a longitudinal direction thereof and has an outer diameter
greater than that of the remaining portion, and a fixation assisting
member is provided on the rear end side of the collar portion in order to
assist a function of the metallic shell of holding the insulator; andthe
fixation assisting member includes at least a high dielectric constant
material which is higher in dielectric constant than alumina.

2. (canceled)

3. (canceled)

4. A spark plug according to claim 1, wherein the high dielectric constant
material is applied to an outer surface of the insulator as a coating
layer.

5. A spark plug according to claim 1 or claim 4, wherein the high
dielectric constant material contains an ABO3-type perovskite oxide,
wherein "A" is at least one of Ca, Sr, Ba, Pb, and La; and "B" site is at
least one of Zr, Ti, Ce, and Al).

6. A spark plug according to claim 1 or claim, wherein the high dielectric
constant material contains an oxide of zirconium (Zr) or hafnium (Hf).

7. A spark plug comprising:a center electrode,a terminal portion
electrically connected to the center electrode so as to apply a voltage
from an external power source to the center electrode;a ground electrode
disposed on a front end side such that a gap for spark discharge is
formed between the ground electrode and the center electrode; anda
metallic shell which holds the ground electrode, which is electrically
connected to the ground electrode, and in which the center electrode is
disposed, whereina capacitance between the terminal portion and the
metallic shell is 16.0 pF or greater.

8. A spark plug according to claim 7, wherein the capacitance is 18.0 pF
or greater.

9. A spark plug according to claim 7 or claim 8, wherein the capacitance
is 29.0 pF or greater.

10. A spark plug according to claim 7 or claim 8, wherein the capacitance
is 36.0 pF or greater.

11. A spark plug according to claim 7 or claim 8, wherein the capacitance
is 58.0 pF or less.

12. A spark plug according to any claim 7 or claim 8, wherein the
capacitance is increased by a member which is disposed between the
terminal portion and the metallic shell and which is higher in dielectric
constant than alumina.

Description:

FIELD OF THE INVENTION

[0001]The present invention relates to a spark plug.

BACKGROUND OF THE INVENTION

[0002]A spark plug includes a ground electrode and a center electrode
disposed to face each other with a gap (clearance) therebetween, and
generates spark discharge upon application of high voltage between the
two electrodes. However, it is known that, since spark discharge causes
an instantaneous change in current, radio noise is generated at the time
of ignition by the spark plug. If this radio noise becomes severe, the
noise not only affects electronic devices, such as an ECU (Engine Control
Unit), of a vehicle or the like onto which the spark plug is mounted, but
may also exert electromagnetic interference on the surroundings.
Heretofore, various techniques have been proposed in order to reduce such
radio noise. (See, for example, Japanese Patent Application Laid-Open
(kokai) No. S61-135079).

[0003]However, since radio noise generated in a spark plug includes radio
waves in a wide frequency range from a low frequency to a high frequency,
in actuality, conventional techniques, including the above-mentioned
prior art technique, cannot reduce the radio noise sufficiently.

SUMMARY OF THE INVENTION

[0004]An advantage of the present invention is a technique for suppressing
the generation of radio noise in a spark plug.

[0005]The present invention has been accomplished in order to at least
partially solve the above-described problem, and can be realized in the
following modes or application examples.

[0006]In accordance with one aspect of the present invention, there is
provided a spark plug comprising:

[0007]a center electrode,

[0008]a terminal portion electrically connected to the center electrode so
as to apply a voltage from an external power source to the center
electrode;

[0009]a ground electrode disposed on a front end side such that a gap for
spark discharge is formed between the ground electrode and the center
electrode; and

[0010]a metallic shell which holds the ground electrode, which is
electrically connected to the ground electrode, and in which the center
electrode is disposed, wherein

[0011]a high dielectric constant material which is higher in dielectric
constant than alumina is disposed between the terminal portion and the
metallic shell.

[0012]In general, the capacitance between the terminal portion and the
metallic shell serves as a capacitor which attenuates the voltage of
discharge current flowing toward the terminal portion. In the
above-described structure, since the capacitance of this capacitor is
increased by the high dielectric constant material, the degree of
attenuation of the discharge current can be increased. Accordingly, the
generation of radio noise in the spark plug can be suppressed.

[0013]In accordance with a second aspect of the present invention, there
is provided a spark plug as described above, further comprising an
insulator which is held within the metallic shell and in which the center
electrode and the terminal portion are held, wherein

[0014]the high dielectric constant material is disposed between the
insulator and the metallic shell.

[0015]According to this spark plug, the high dielectric constant material
disposed between the insulator and the metallic shell can suppress the
generation of radio noise in the spark plug.

[0016]In accordance with a third aspect of the present invention, there is
provided a spark plug as described above, wherein the insulator includes
a collar portion which is located at an approximate center with respect
to a longitudinal direction thereof and has an outer diameter greater
than that of the remaining portion, and a fixation assisting member is
provided on the rear end side of the collar portion in order to assist a
function of the metallic shell of holding the insulator; and

[0017]the fixation assisting member includes at least the high dielectric
constant material.

[0018]Conventionally, in a spark plug, talc powder is used as a fixation
assisting member. A powder material which is higher in dielectric
constant than alumina can be used to be mixed in the talc powder or
replace the talc powder. By virtue of this configuration, the bonding
between the insulator and the metallic shell can be maintained, and the
generation of radio noise in the spark plug can be suppressed.

[0019]In accordance with a fourth aspect of the present invention, there
is provided a spark plug as described above, wherein

[0020]the high dielectric constant material is applied to an outer surface
of the insulator as a coating layer.

[0021]Conventionally, in a spark plug, a glaze is applied to cover the
outer surface of the insulator in order to increase the strength of the
insulator. A material which is higher in dielectric constant than alumina
can be mixed into the glaze or can be used to replace the glaze so as to
form a coating layer on the outer surface of the insulator. By virtue of
this configuration, the strength of the insulator can be maintained, and
the generation of radio noise in the spark plug can be suppressed.
Notably, the coating layer is not necessarily required to be provided on
the outermost surface of the insulator, and another coating layer may be
formed on the outer side of the former coating layer.

[0022]In accordance with a fifth aspect of the present invention, there is
provided a spark plug as described above, wherein the high dielectric
constant material contains an ABO3-type perovskite oxide (A site is at
least one of Ca, Sr, Ba, Pb, and La; and B site is at least one of Zr,
Ti, Ce, and Al).

[0023]In accordance with a sixth aspect of the present invention, there is
provided a spark plug as described above, wherein the high dielectric
constant material contains an oxide of zirconium (Zr) or hafnium (Hf).

[0024]In accordance with a seventh aspect of the present invention, there
is provided a spark plug comprising:

[0025]a center electrode,

[0026]a terminal portion electrically connected to the center electrode so
as to apply a voltage from an external power source to the center
electrode;

[0027]a ground electrode disposed on a front end side such that a gap for
spark discharge is formed between the ground electrode and the center
electrode; and

[0028]a metallic shell which holds the ground electrode, which is
electrically connected to the ground electrode, and in which the center
electrode is disposed, wherein

[0029]a capacitance between the terminal portion and the metallic shell is
16.0 pF or greater.

[0030]In general, the capacitance between the terminal portion and the
metallic shell serves as a capacitor which attenuates the voltage of
discharge current flowing toward the terminal portion. In the
above-described structure, since the capacitance of this capacitor is
16.0 pF or greater, the degree of attenuation of the discharge current
can be increased. Accordingly, the generation of radio noise in the spark
plug can be suppressed.

[0031]In accordance with an eighth aspect of the present invention, there
is provided a spark plug as described with respect to the seventh aspect,
wherein the capacitance is 18.0 pF or greater.

[0032]According to this spark plug, since the capacitance between the
terminal portion and the metallic shell is increased further, the
generation of radio noise can be suppressed to a greater extent.

[0033]In accordance with a ninth aspect of the present invention, there is
provided a spark plug according to the seventh and eighth aspect of the
present invention, wherein the capacitance is 29.0 pF or greater.

[0034]According to this spark plug, since the capacitance between the
terminal portion and the metallic shell is increased further, the
generation of radio noise can be suppressed to a greater extent.

[0035]In accordance with a tenth aspect of the present invention, there is
provided a spark plug according to the seventh, eighth and ninth aspects
of the present invention, wherein the capacitance is 36.0 pF or greater.

[0036]According to this spark plug, since the capacitance between the
terminal portion and the metallic shell is increased further, the
generation of radio noise can be suppressed to a greater extent.

[0037]In accordance with an eleventh aspect of the present invention,
there is provided a spark plug according to the seventh through tenth
aspects of the present invention, wherein the capacitance is 58.0 pF or
less.

[0038]According to this spark plug, the generation of radio noise can be
suppressed effectively.

[0039]In accordance with a twelfth aspect of the present invention, there
is provided a spark plug according to the seventh through eleventh
aspects of the present invention, wherein the capacitance is increased by
a member which is disposed between the terminal portion and the metallic
shell and which is higher in dielectric constant than alumina.

[0040]According to this spark plug, the capacitance between the terminal
portion and the metallic shell can be increased by increasing the
dielectric constant of the member disposed between the terminal portion
and the metallic shell. Accordingly, the generation of radio noise in the
spark plug can be suppressed.

[0041]Notably, the present invention can be realized in various forms; for
example, the invention can be realized in the form of a spark plug, in
the form of an internal combustion engine onto which the spark plug is
mounted, or in the form of a vehicle onto which the internal combustion
engine is mounted.

BRIEF DESCRIPTION OF THE DRAWINGS

[0042]FIG. 1 is a schematic cross-sectional view showing the structure of
a spark plug according to a first embodiment.

[0043]FIG. 2 is a schematic cross-sectional view of the spark plug
according to the first embodiment, and circuit diagram showing its
equivalent circuit.

[0044]FIG. 3 is a graph showing an effect of suppressing radio noise by a
fixation assisting member.

[0045]FIG. 4 is a schematic cross-sectional view of a spark plug according
to a second embodiment, and circuit diagram showing its equivalent
circuit.

[0046]FIG. 5 is a schematic cross-sectional view of a spark plug according
to a third embodiment, and circuit diagram showing its equivalent
circuit.

[0047]FIG. 6 is an explanatory table showing radio noise suppression
effects of spark plugs according to a fourth embodiment.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0048]Next, embodiments of the present invention will be described in the
following sequence.

[0049]A. First embodiment:

[0050]B. Second embodiment:

[0051]C. Third embodiment:

[0052]D. Fourth embodiment:

[0053]E. Modifications:

A. First Embodiment

[0054]FIG. 1 is a schematic diagram showing the structure of a spark plug
according to one embodiment of the present invention. The spark plug 100
includes an insulator 10, a center electrode 20, a ground electrode 30, a
metallic shell 40, and a metallic terminal 50. This spark plug 100 is
attached to a combustion chamber of an internal combustion engine, and
generates spark discharge between two electrodes (the center electrode 20
and the ground electrode 30) disposed to form a gap GP therebetween.

[0055]The insulator 10 is an insulating member which constitutes a body
portion of the spark plug 100 for holding the two electrodes 20 and 30.
For example, the insulator 10 is formed by firing insulating ceramic such
as alumina (Al2O3). The insulator 10 assumes a tubular shape,
and includes an axial hole 12 extending along the direction of an axis O
shown in FIG. 1. The insulator 10 has a collar portion 15 which is formed
at an approximate center thereof with respect to the direction of the
axis O and at which the insulator 10 has the maximum outer diameter.
Notably, the outer surface of the insulator 10 is covered by a glaze
layer 11 formed through application of glaze. The glaze layer 11
increases the strength of the insulator 10.

[0056]On the rear end side of the insulator 10, a metallic terminal 50,
which is electrically connected to an external power source, is inserted
into an opening portion 13 of the axial hole 12, and is fixedly held
therein. This metallic terminal 50 corresponds to the "terminal portion"
of the present invention. Further, on the side of the insulator opposite
the side where the metallic terminal 50 is disposed, a center electrode
20 is inserted into an opening portion 14 of the axial hole 12, and is
fixedly held therein. Two seal portions 60a and 60b, and a resistor 70
are provided within the axial hole 12 between the center electrode 20 and
the metallic terminal 50. In the following description, the side where
the center electrode 20 is disposed will be referred to as the "front end
side."

[0057]The first seal portion 60a is provided between the metallic terminal
50 and the resistor 70, and the second seal portion 60b is provided
between the resistor 70 and the center electrode 20. The two seal
portions 60a and 60b fix the metallic terminal 50 and the center
electrode 20 to the wall surface of the axial hole 12, establish
electrical continuity therebetween, and secure airtightness within the
axial hole 12. The seal portions 60a and 60b are formed of a glass
material having electrical conductivity. The function of the resistor 70
will be described later. Notably, the resistor 70 desirably has a
resistance (for example, about 5 kΩ) which does not affect the
igniting performance of the spark plug 100.

[0058]A portion of the insulator 10 which extends from the collar portion
15 toward the opening portion 13, into which the metallic terminal 50 is
inserted, will be referred to as a "terminal-side tube portion 16".
Further, the insulator 10 has a step 19 provided on a portion extending
from the collar portion 15 toward the center electrode 20, whereby two
portions having different diameters are formed. Hereinafter, a larger
diameter portion extending from the collar portion 15 to the step 19 will
be referred to as an "electrode-side tube portion 17," and a smaller
diameter portion extending from the step 19 to the opening portion 14,
into which the center electrode 20 is inserted, will be referred to as a
"front-end tube portion 18."

[0059]A metallic shell 40, which is an approximately cylindrical metallic
member, is disposed around the insulator 10. More specifically, the
metallic shell 40 accommodates a portion of the terminal-side tube
portion 16, the collar portion 15, the electrode-side tube portion 17,
the step 19, and a portion of the front-end tube portion 18 of the
insulator 10, and holds the insulator 10 by means of crimping (the
crimping will be described later). A ground electrode 30 is provided at a
front end portion 43 of the metallic shell 40, which is a portion thereof
located on the front end side. The ground electrode 30 is bent into an
approximately L-like shape. One end of the ground electrode 30 is welded
to the metallic shell 40, and the other end thereof faces a front end
portion 21 of the center electrode 20 via a gap GP (hereinafter referred
to as the "spark gap GP").

[0060]The metallic shell 40 has a crimp portion 41, whose wall thickness
is rendered relative small in order to facilitate crimping work, which is
a process for holding the insulator 10. The crimp portion 41 is provided
at the rear end of the metallic shell 40, and is bent inward in order to
urge the collar portion 15 toward the front end via a fixation assisting
portion 80, which will be described later. Meanwhile, on the front end
side of the metallic shell 40, a step 12b is formed on the inner
circumference of the metallic shell 40 by means of reducing the diameter
of the axial hole 12. The step 12b receives the step 19 of the insulator
10 to thereby establish an airtight state. In order to improve the
airtightness, a plate packing may be interposed between the step 12b and
the step 19 as well known.

[0061]Notably, a tool engagement portion 44, dimensioned to engage a spark
plug wrench, is provided on the front end side of the crimp portion 41.
Further, the metallic shell 40 has a screw portion 42, which is used to
fix the spark plug 100 to a mount portion of an internal combustion
engine through screw engagement.

[0062]The fixation assisting portion 80 is provided between the crimp
portion 41 and the collar portion 15 in order to assist fixing between
the insulator 10 and the metallic shell 40. Specifically, at the rear end
of the metallic shell 40, a space is formed between the inner
circumferential surface of the metallic shell 40 and the outer surface of
the insulator 10. A high dielectric constant fixation assisting member 81
and two wire packings 82 and 83 are disposed in the space. Thus, the
fixation assisting portion 80 is formed. More specifically, the high
dielectric constant fixation assisting member 81, which is a ring-shaped
powder compact formed through pressing of powder, is disposed between the
two wire packings 82 and 83, which surround the outer circumference of
the insulator 10.

[0063]The fixation assisting portion 80 functions as a cushioning material
for absorbing differences in thermal expansion among the constituent
members of the spark plug 100 and impact forces acting on the insulator
10. The fixation assisting portion 80 also has a function of improving
the airtightness between the insulator 10 and the metallic shell 40. The
high dielectric constant fixation assisting member 81 is formed of a high
dielectric constant material which is higher in dielectric constant
(relative dielectric constant) than Al2O3, which is the main
component of the insulator 10. By way of example and not limitation, the
high dielectric constant fixation assisting member 81 can be formed of
barium titanate (BaTiO3). Notably, the dielectric constant of
Al2O3 is about 8 to 11, and the dielectric constant of
BaTiO3 is about 100 to 1000, although it changes with temperature.
The reason why the high dielectric constant fixation assisting member 81
is formed of a material whose dielectric constant is higher than that of
Al2O3 will be described later.

[0064]FIGS. 2(A) and 2(B) are an explanatory view and an explanatory
diagram, respectively, used for explaining a mechanism by which radio
noise is suppressed in the spark plug 100. FIG. 2(A) is a schematic
cross-sectional view showing the structure of the spark plug 100. FIG.
2(A) is identical with FIG. 1, except that cross-hatching and symbols are
changed in order to facilitate description and understanding. Here, there
is assumed a case where spark discharge is generated at the spark gap GP.
At that time, a discharge current Is flows to the outside of the spark
plug 100 via the center electrode 20, the second seal portion 60b, the
resistor 70, the first seal portion 60a, and the metallic terminal 50, in
this sequence.

[0065]Here, a cross-hatched area which includes the center electrode 20
and the second seal portion 60b and which extends to the resistor 70 will
be referred to as a "first conductive portion CP1." Further, a
cross-hatched area which includes the first seal portion 60a and which
extends from the resistor 70 to a portion of the metallic terminal 50
inserted into the axial hole 12 of the insulator 10 will be referred to
as a "second conductive portion CP2." Meanwhile, a cross-hatched area
which includes the ground electrode 30 and a portion of the metallic
shell 40 which faces the first conductive portion CP1 via a portion of
the insulator 10 will be referred to as a "first ground electrode GE1"
Further, a portion of the metallic shell 40 which extends from the first
ground electrode GE1 toward the metallic terminal 50 side will be
referred to as a "second ground electrode GE2."

[0066]The first conductive portion CP1 and the first ground electrode GE1
can be considered to constitute a capacitor by sandwiching a portion of
the insulator 10, which is a dielectric material. Similarly, the second
conductive portion CP2 and the second ground electrode GE2 can be
considered to constitute a capacitor; and the first and second seal
portions 60a and 60b can be considered to constitute a capacitor by
sandwiching the resistor 70. Therefore, when the spark plug 100 generates
radio noise, it can be considered to form an electric circuit as
described below.

[0067]FIG. 2(B) is a circuit diagram showing an equivalent circuit 200 of
the spark plug 100 at the time when it generates radio noise. An AC power
source 201 corresponds to the spark gap GP, which is generating spark
discharge. Accordingly, an input voltage Eg provided from the AC power
source 201 is equal to a discharge voltage of the spark plug 100. A first
resistor 202 corresponds to a resistor at the spark gap GP through which
the discharge current Is flows (hereinafter referred to as a "discharge
resistor"). Notably, the resistance of the first resistor 202 is
represented by rg. A second resistor 205 connected in series to the first
resistor 202 corresponds to the resistor 70 of the spark plug 100. The
resistance of the second resistor 205 is represented by Rr.

[0068]A first capacitor 211 is provided in a first ground path 203 which
extends from a line between the first and second resistors 202 and 205,
and is connected to the ground. The first capacitor 211 corresponds to a
capacitor formed by the above-described first conductive portion CP1 and
first ground electrode GE1. The capacitance of the first capacitor 211 is
represented by Cg.

[0069]In the equivalent circuit 200, a second capacitor 213 is connected
in parallel to the second resistor 205. The second capacitor 213
corresponds to a capacitor formed by the above-described first and second
seal portions 60a and 60b. The capacitance of the second capacitor 213 is
represented by Cr.

[0070]In the equivalent circuit 200, a second ground path 207 extends from
a line located the output side of the second resistor 205, and is
connected to the ground. A third capacitor 215 is provided in the second
ground path 207. The third capacitor 215 corresponds to a capacitor
formed by the above-described second conductive portion CP2 and second
ground electrode GE2. The capacitance of the third capacitor 215 is
represented by Cu.

[0071]A voltage ratio A, which the ratio between the input voltage Eq and
the output voltage Es in the equivalent circuit 200, can be obtained from
the above-described resistances rg, Rr and capacitances Cg, Cr, Cu in
accordance with the following Equation (1). Further, the voltage
attenuation S in the equivalent circuit 200 can be obtained from the
voltage ratio A in accordance with the following Equation (2).

[0072]In Equation (1), the coefficient Zo represents the
characteristic impedance of an external cable 220 connected to the output
side of the equivalent circuit 200.

[0073]The greater the attenuation S of Equation (2), the greater the
degree to which radio noise is reduced in the spark plug 100. The
inventors of the present invention found that the attenuation S can be
increased by increasing the value of the capacitance Cu in Equation (1).
The capacitance Cu can be increased by increasing the dielectric constant
between the second conductive portion CP2 and the second ground electrode
GE2 shown in FIG. 2(A). In particular, the capacitance Cu can be
increased efficiently by increasing the dielectric constant of the member
disposed between the second conductive portion CP2 and an end portion
(rear-end-side portion) 40e of the metallic shell 40 including the crimp
portion 41 and the tool engagement portion 44. In the present embodiment,
the high dielectric constant fixation assisting member 81, which is
higher in dielectric constant than Al2O3 (the main component of
the insulator 10), is provided between the second conductive portion CP2
and the second ground electrode GE2, whereby radio noise generated from
the spark plug 100 is reduced.

[0074]FIGS. 3(A) and 3(B) are graphs showing the radio noise suppression
effect of the high dielectric constant fixation assisting member 81, in
which change in attenuation with the frequency of radio noise is shown.
FIG. 3(A) is a graph showing the result of simulation on attenuation of
radio noise in a spark plug, which is obtained from the above-described
Equations (1) and (2). Specifically, curve G1 shows the result of
simulation which was performed, with the dielectric constant of the high
dielectric constant fixation assisting member 81 set to 1000 for an
assumed case where the high dielectric constant fixation assisting member
81 is formed of BaTiO3. Curve G2 shows the result of simulation
which was performed, with the dielectric constant of the high dielectric
constant fixation assisting member 81 set to 2 for an assumed case where
the high dielectric constant fixation assisting member 81 is formed of
talc only (Comparative Example).

[0075]FIG. 3(B) is a graph showing actual values of attenuation of radio
noise measured by the inventors of the present invention. A BOX method
(JASO D002-2: 2004) was employed so as to measure the attenuation of
radio noise. Curve G1a shows the attenuation of radio noise of a spark
plug in which the high dielectric constant assisting member 81 is formed
of BaTiO3 (dielectric constant: 1000). Broken curve G2a shows the
attenuation of radio noise of a spark plug in which the high dielectric
constant assisting member 81 is formed of talc (dielectric constant: 2)
only (Comparative Example). Moreover, single-dot curve G3 shows the
attenuation of radio noise of a spark plug in which the high dielectric
constant assisting member 81 is formed of
Ba0.9Sr0.1Ti0.85Zr0.15O3 (dielectric constant:
1800).

[0076]As shown in these graphs, in both the results of simulation and the
actually measured values, the spark plug in which the high dielectric
constant fixation assisting member 81 is formed of a material whose
dielectric constant is higher than that of alumina is greater in
attenuation than the spark plug of Comparative Example. Further,
comparison between the curves G1a and G3 reveals that the higher the
dielectric constant of the high dielectric constant fixation assisting
member 81, the greater the attenuation attained thereby, and the greater
the radio noise suppression effect.

[0077]In general, a spark plug includes a fixation assisting member which
is formed through press forming of powder and which is provided at a
position similar to that of the high dielectric constant fixation
assisting member 81 of the spark plug 100 of the present embodiment.
Below is described a method of measuring the dielectric constant of the
fixation assisting member.

[0078](i) The volume of the fixation assisting member is measured.
Specifically, the volume of the fixation assisting member may be obtained
by measuring the cross section of the fixation assisting member by use of
a plurality of cross-sectional images of a spark plug obtained through
radiography, and calculating the volume from the measured cross section.
Alternatively, the volume of the fixation assisting member may be
obtained by actually cutting the spark plug and the fixation assisting
member.

[0079](ii) The weight of the fixation assisting member is measured.
Specifically, the weight of the fixation assisting member, which is
removed from the spark plug through disassembly, may be measured.

[0080](iii) The charging density of powder which constitutes the fixation
assisting member is calculated from the results of the measurements in
the above-described steps (i) and (ii).

[0081](iv) A measurement sample for measuring dielectric constant is
prepared. Specifically, a charging pressure is calculated from the
charging density calculated in the above-described step (iii), and a
separately prepared material powder having the same composition as the
fixation assisting member is press-formed under that charging pressure,
whereby the measurement sample is prepared. Notably, the material powder
may be powder of the fixation assisting member collected by disassembling
a plurality of spark plugs of the same type.

[0082](v) The dielectric constant of the prepared measurement sample is
measured by a parallel-conductor-plate-type dielectric resonator method
based on JIS R1627 (1996).

[0083]This measurement method can determine the dielectric constant of the
fixation assisting member.

[0084]As described above, radio noise of the spark plug 100 can be reduced
by means of disposing between the second conductive portion CP2 and the
metallic shell 40 a high dielectric constant material whose dielectric
constant is higher than that of alumina.

B. Second Embodiment

[0085]FIG. 4(A) is a schematic cross-sectional view showing the structure
of a spark plug 100B according to a second embodiment of the present
invention. FIG. 4(A) is generally the same as FIG. 2(A), except that a
high dielectric constant coating layer 90 is provided on the outer
surface of the insulator 10. FIG. 4(B) is a circuit diagram showing an
equivalent circuit 200 of the spark plug 100B, and is generally the same
as FIG. 2(B).

[0086]In this spark plug 100B, the high dielectric constant coating layer
90, which is formed through application of BaTiO3 which is a high
dielectric constant material, is further provided on the outer surface of
the glaze layer 11 of the insulator 10. The high dielectric constant
coating layer 90 covers a region indicated by a broken line.
Specifically, the high dielectric constant coating layer 90 covers a
portion of the outer surface of the insulator 10, which portion includes
the outer surface of the terminal-side tube portion 16 and extends to a
wall surface 15w of the collar portion 15 which constitutes a wall
surface of the fixation assisting portion 80.

[0087]As described above, even in the case where a coating layer of a high
dielectric constant material is provided on the outer surface of the
insulator 10 located between the metallic shell 40 and the second
conductive portion CP2, the capacitance Cu of the third capacitor 215 can
be increased. Accordingly, radio noise of the spark plug 100B can be
reduced further.

C. Third Embodiment

[0088]FIG. 5(A) is a schematic cross-sectional view showing the structure
of a spark plug 100C according to a third embodiment of the present
invention. FIG. 5(A) is generally the same as FIG. 4(A), except that a
high dielectric constant insulator 100 is used in place of the insulator
10. FIG. 5(B) is a circuit diagram showing an equivalent circuit 200 of
the spark plug 100C, and is generally the same as FIG. 4(B).

[0089]The high dielectric constant insulator 10C of this spark plug 100C
is formed of Al2O3 into which BaTiO3 is mixed as a high
dielectric constant material. Preferably, BaTiO3 having an average
grain size of 5 μm or greater is used so as to suppress melting into
glass at the time of firing.

[0090]As described above, even in the case where a material which is
higher in dielectric constant than Al2O3 is mixed into the
material of the insulator, the capacitance Cu of the third capacitor 215
can be increased. Accordingly, radio noise of the spark plug 100C can be
reduced further.

D. Fourth Embodiment

[0091]FIG. 6 is an explanatory table showing radio noise suppression
effects of spark plugs according to a fourth embodiment of the present
inventions. In this fourth embodiment, spark plugs of six types were
assumed as Examples, and spark plugs of two types were assumed as
Comparative Examples; and, for each spark plug, a predicted attenuation
of radio noise was calculated through simulation. FIG. 6 is a table
showing the results of the calculation. Specifically, this table shows
the capacitances Cu, Cg, Cr of the first through third capacitors 211,
213, 215, and the resistance rg, Rr of the first and second resistors
202, 205 in each of the spark plugs of Comparative Examples and Examples.

[0092]Further, the table of FIG. 6 shows the calculated radio noise
attenuation at a frequency of 500 MHz for each of the spark plugs of
Comparative Examples and Examples, and the ratio (effect ratio) of the
radio noise attenuation of each of Comparative Examples and Examples to
that of each Comparative Example. Notably, the frequency of 500 MHz at
which the attenuation was calculated was selected as a representative
frequency of an intermediate frequency band and a high frequency band in
which the effect of the present invention appears remarkably.

[0093]Moreover, the table of FIG. 6 shows the results of evaluation on the
radio noise suppression effects of Comparative Examples and Examples
performed on the basis of their effect ratios, in which the evaluation
results are indicated by "Poor," "Good," and "Excellent." Specifically,
when one of the effect ratio of a certain Example to Comparative Example
1 and the effect ratio of the certain Example to Comparative Example 2
was less than 1.1, the certain Example was evaluated as "Poor." When both
the effect ratios of the certain Example were equal to or greater than
1.1 and one of the effect ratios of the certain Example was not greater
than 1.3, the certain Example was evaluated as "Good." When both the
effect ratios of the certain Example were equal to or greater than 1.3,
the certain Example was evaluated as "Excellent."

[0094]The spark plug of Example 1 has a structure similar to that of the
spark plug 100 of the first embodiment (FIG. 1), and includes the high
dielectric constant fixation assisting member 81. The spark plug of
Example 2 has a structure similar to that of the spark plug 100B of the
second embodiment (FIG. 4), except that the spark plug of Example 2
includes talc instead of the high dielectric constant assisting member
81. The spark plug of Example 2 includes the high dielectric constant
coating layer 90. The spark plug of Example 3 has a structure similar to
that of the spark plug 100B of the second embodiment, and includes both
the high dielectric constant fixation assisting member 81 and the high
dielectric constant coating layer 90.

[0095]The spark plug of Example 4 includes the high dielectric constant
fixation assisting member 81, as in the case of the spark plug of Example
1. The spark plug of Example 5 does not include the high dielectric
constant fixation assisting member 81, but includes the high dielectric
constant coating layer 90, as in the case of the spark plug of Example 2.
The spark plug of Example 6 includes both the high dielectric constant
fixation assisting member 81 and the high dielectric constant coating
layer 90, as in the case of the spark plug of Example 3.

[0096]Meanwhile, each of the spark plugs of Comparative Example 1 and
Comparative Example 2 has a structure similar to those of conventional
spark plugs; that is, each of the spark plugs of Comparative Example 1
and Comparative Example includes talc instead of the high dielectric
constant fixation assisting member 81 and does not include the high
dielectric constant coating layer 90. However, the spark plug of
Comparative Example 1 and the spark plug of Comparative Example 2 differ
from each other in terms of the capacitances Cu, Cg, Cr and the
resistances rg, Rr. Notably, the spark plug of Comparative Example 1
differs from the spark plugs of Example 1 to Example 3 only in the value
of the capacitance Cu, and the remaining capacitances Cg, Cr and the
resistances rg, Rr of the spark plug of Comparative Example 1 are the
same as those of the spark plugs of Example 1 to Example 3. Further, the
spark plug of Comparative Example 2 differs from the spark plugs of
Example 4 to Example 6 only in the value of the capacitance Cu, and the
remaining capacitances Cg, Cr and the resistances rg, Rr of the spark
plug of Comparative Example 2 are the same as those of the spark plugs of
Example 4 to Example 6.

[0097]Comparison between the evaluation results of Examples 1 and 2 and
that of Example 3 reveals that a better evaluation result was attained in
Example 3 whose capacitance Cu is large. Similarly, comparison between
the evaluation results of Examples 4 and 5 and that of Example 6 reveals
that a better evaluation result was attained in Example 6 whose
capacitance Cu is large. The above indicates that the greater the
capacitance Cu, the higher the radio noise suppression effect, which is
preferred. More specifically, the lower limit of the capacitance Cu is
preferably 16.0 pF or greater, more preferably 18.0 pF or greater. Also,
the lower limit of the capacitance Cu is preferably 25.0 pF or greater,
more preferably 29.0 pF or greater. Further, the lower limit of the
capacitance Cu is preferably 30.0 pF or greater, more preferably 36.0 pF
or greater. Notably, the upper limit of the capacitance Cu is preferably
58.0 pF or less, more preferably 40.0 pF or less.

[0098]Further, a spark plug including both the high dielectric constant
assisting member 81 and the high dielectric constant coating layer 90 is
preferred, because the capacitance Cu can be increased further. Notably,
the spark plug may be configured such that another high dielectric
constant material (e.g., the high dielectric constant insulator 10C of
the third embodiment) is disposed between the second conductive portion
CP2 and the metallic shell 40 so as to increase the capacitance Cu.

[0099]The capacitance Cu can be increased by using a material having a
higher dielectric constant as the high dielectric constant material
disposed between the second conductive portion CP2 and the metallic shell
40, such as the high dielectric constant fixation assisting member 81, or
the high dielectric constant coating layer 90. Further, the capacitance
Cu can be increased by changing the structures of the second conductive
portion CP2 and the metallic shell 40. Specifically, the capacitance Cu
can be increased by increasing the surface areas of the second conductive
portion CP2 and the metallic shell 40, or by decreasing the distance
between the second conductive portion CP2 and the metallic shell 40.
Moreover, the capacitance Cu can be increased by increasing the ratio of
occupation of the high dielectric constant material within the space
between the second conductive portion CP2 and the metallic shell 40. More
specifically, the capacitance Cu can be increased by increasing the
volume of the high dielectric constant fixation assisting member 81 or
the thickness of the high dielectric constant coating layer 90.

[0100]As described above, according to the spark plugs of the fourth
embodiment, radio noise of the spark plugs can be reduced further by
increasing the value of the capacitance Cu.

E. Modifications

[0101]Notably, the present invention is not limited to the above-described
examples and embodiments, and may be practiced in various forms without
departing from the scope of the invention. For example, the following
modifications are possible.

[0102]E1. Modification 1:

[0103]In the above-described embodiments, the high dielectric constant
fixation assisting member 81, the high dielectric constant coating layer
90, and/or the high dielectric constant insulator 10C is formed of a high
dielectric constant material. However, the embodiments may be modified
such that other portions are formed of a high dielectric constant
material. No limitation is imposed on the position of the high dielectric
constant material, so long as the high dielectric constant material is
provided between the second conductive portion CP2 and the metallic shell
40. For example, the wire packings 82 and 83 may be formed of a high
dielectric constant material. In this case, the high dielectric constant
fixation assisting member 81 may be omitted. Further, the second
embodiment may be modified such that the high dielectric constant
fixation assisting member 81 is omitted, and only the high dielectric
constant coating layer 90 is provided; and the third embodiment may be
modified such that the high dielectric constant fixation assisting member
81 and/or the high dielectric constant coating layer 90 is omitted.

[0104]E2. Modification 2:

[0105]In the above-described embodiments, BaTiO3 is employed as a
high dielectric constant material; however, other high dielectric
constant materials may be employed. Preferred high dielectric constant
materials include ABO3-type perovskite oxides (The variable "A" is
comprised of at least one of Ca, Sr, Ba, Pb, and La, and the variable "B"
is comprised of at least one of Zr, Ti, Ce, and Al), zirconium (Zr)
oxide, and hafnium (Hf) oxide.

[0106]E3. Modification 3:

[0107]In the above-described embodiments, the high dielectric constant
fixation assisting member 81 is formed of a powder compact; however, the
high dielectric constant fixation assisting member 81 is not necessarily
required to be formed of a powder compact. However, in the case where the
high dielectric constant fixation assisting member 81 is formed of a
powder compact, the high dielectric constant fixation assisting member 81
can also function as a cushioning member in the spark plug 100 more
effectively.

[0108]E4. Modification 4:

[0109]In the above-described second and third embodiments, on the outer
surface of the insulator 10B, 10C, the glaze layer 11 is overlaid with
the high dielectric constant coating layer 90. However, the second and
third embodiments may be modified such that, in place of the glaze layer
11, a high dielectric constant glaze layer formed through application of
a glaze into which a material whose dielectric constant is higher than
that of Al2O3 is mixed is provided so as to increase the
dielectric constant of the outer surface of the insulator.

[0110]Notably, the dielectric constant of the coating layer provided on
the outer surface of the insulator can be determined by the following
method. That is, the composition of the coating layer is determined by
use of an electron probe microanalyzer (EPMA: Electron Probe Micro
Analysis), and the dielectric constant is calculated from the
composition. Notably, as this dielectric constant calculation method, the
A. A. Appen method (reference: Chemistry of Glass (1974) published by
Nisso Tsuushin Sha and written by A. A. Appen) can be used.